Searcharxiv⌕ Search

arXiv subjects

Bergfinnur Durhuus

Publications and source records attributed to Bergfinnur Durhuus.

29 records · Page 2Linked to original sources
arXiv2005

Random walks on combs

We develop techniques to obtain rigorous bounds on the behaviour of random walks on combs. Using these bounds we calculate exactly the spectral dimension of random combs with infinite teeth at random positions or teeth with random but finite length. We also calculate exactly the spectral dimension of some fixed non-translationally invariant combs. We relate the spectral dimension to the critical exponent of the mass of the two-point function for random walks on random combs, and compute mean displacements as a function of walk duration. We prove that the mean first passage time is generally infinite for combs with anomalous spectral dimension.

hep-th↗
arXiv2005

On the entropy of LEGO

We propose the further study of the rate of growth of the number of contiguous buildings which may be made from n LEGO blocks of the same size and color. Specializing to blocks of dimension 2x4 we give upper and lower bounds, and speculate on the true value.

math.CO↗
arXiv2004

Noncommutative waves have infinite propagation speed

We prove the existence of global solutions to the Cauchy problem for noncommutative nonlinear wave equations in arbitrary even spatial dimensions where the noncommutativity is only in the spatial directions. We find that for existence there are no conditions on the degree of the nonlinearity provided the potential is positive. We furthermore prove that nonlinear noncommutative waves have infinite propagation speed, i.e., if the initial conditions at time 0 have a compact support then for any positive time the support of the solution can be arbitrarily large.

hep-th↗
arXiv2002

A note on noncommutative scalar multisolitons

We prove that there do not exist multisoliton solutions of noncommutative scalar field theory in the Moyal plane which interpolate smoothly between $n$ overlapping solitons and $n$ solitons with an infinite separation.

hep-th↗
arXiv2001

The Existence and Stability of Noncommutative Scalar Solitons

We establish existence and stabilty results for solitons in noncommutative scalar field theories in even space dimension $2d$. In particular, for any finite rank spectral projection $P$ of the number operator ${\mathcal N}$ of the $d$-dimensional harmonic oscillator and sufficiently large noncommutativity parameter $θ$ we prove the existence of a rotationally invariant soliton which depends smoothly on $θ$ and converges to a multiple of $P$ as $θ\to\infty$. In the two-dimensional case we prove that these solitons are stable at large $θ$, if $P=P_N$, where $P_N$ projects onto the space spanned by the $N+1$ lowest eigenstates of ${\mathcal N}$, and otherwise they are unstable. We also discuss the generalisation of the stability results to higher dimensions. In particular, we prove stability of the soliton corresponding to $P=P_0$ for all $θ$ in its domain of existence. Finally, for arbitrary $d$ and small values of $θ$, we prove without assuming rotational invariance that there do not exist any solitons depending smoothly on $θ$.

hep-th↗
arXiv1998

A polymer gas on a random surface

Using the observation that configurations of N polymers with hard core interactions on a closed random surface correspond to random surfaces with N boundary components we calculate the free energy of a gas of polymers interacting with fully quantized two-dimensional gravity. We derive the equation of state for the polymer gas and find that all the virial coefficients beyond the second one vanish identically.

hep-th↗
arXiv1996

Branched polymers on branched polymers

We study an ensemble of branched polymers which are embedded on other branched polymers. This is a toy model which allows us to study explicitly the reaction of a statistical system on an underlying geometrical structure, a problem of interest in the study of the interaction of matter and quantized gravity. We find a phase transition at which the embedded polymers begin to cover the basis polymers. At the phase transition point the susceptibility exponent $γ$ takes the value 3/4 and the two-point function develops an anomalous dimension 1/2.

hep-th↗
arXiv1994

Remarks on the entropy of 3-manifolds

We give a simple combinatoric proof of an exponential upper bound on the number of distinct 3-manifolds that can be constructed by successively identifying nearest neighbour pairs of triangles in the boundary of a simplicial 3-ball and show that all closed simplicial manifolds that can be constructed in this manner are homeomorphic to $S^3$. We discuss the problem of proving that all 3-dimensional simplicial spheres can be obtained by this construction and give an example of a simplicial 3-ball whose boundary triangles can be identified pairwise such that no triangle is identified with any of its neighbours and the resulting 3-dimensional simplicial complex is a simply connected 3-manifold.

hep-th↗
arXiv1993

Topological quantum field theory and invariants of graphs for quantum groups

On basis of generalized 6j-symbols we give a formulation of topological quantum field theories for 3-manifolds including observables in the form of coloured graphs. It is shown that the 6j-symbols associated with deformations of the classical groups at simple even roots of unity provide examples of this construction. Calculational methods are developed which, in particular, yield the dimensions of the state spaces as well as a proof of the relation, previously announced for the case of $SU_q(2)$ by V.Turaev, between these models and corresponding ones based on the ribbon graph construction of Reshetikhin and Turaev.

hep-th↗
arXiv1993

Classification and construction of unitary topological field theories in two dimensions

We prove that unitary two-dimensional topological field theories are uniquely characterized by $n$ positive real numbers $λ_1,\ldots λ_n$ which can be regarded as the eigenvalues of a hermitean handle creation operator. The number $n$ is the dimension of the Hilbert space associated with the circle and the partition functions for closed surfaces have the form $$ Z_g=\sum_{i=1}^{n}λ_i^{g-1} $$ where $g$ is the genus. The eigenvalues can be arbitary positive numbers. We show how such a theory can be constructed on triangulated surfaces.

hep-th↗